The present invention relates to a switch monitoring circuit and more particularly to a switch monitoring circuit for supplying a power in response to a command in a state that an ignition is OFF in a vehicle such as a car.
In a vehicle such as a car, there are devices to which a power is to be supplied from a battery mounted on the vehicle also in a sleep state that an ignition is OFF. For example, in the case that a door is unlocked or locked in the sleep state or the case that a door mirror is folded or opened, it is necessary to supply a power from the battery to a motor.
Accordingly, the vehicle has a switch monitoring circuit for monitoring the state of a driving switch for a door lock or a door mirror. The switch monitoring circuit serves to detect the condition of the switch in the sleep state if necessary. Therefore, a microcomputer oscillates a strobe pulse based on a preset timing and a voltage applying unit is turned ON in response to the strobe pulse, thereby applying a power to the switch intermittently. Then, a change in the applied voltage is detected to monitor the opening and closing state of the switch.
Moreover, the switching monitoring circuit has a capacitor for taking a countermeasure against a static electricity to absorb the static electricity between the switch and the microcomputer in order to prevent the hand of an operator from touching the switch to generate the static electricity, and to hinder the microcomputer or the circuit from being damaged due to the static electricity when the operator turns ON/OFF the switch.
The capacitor for taking a countermeasure against the static electricity which is provided in the switch monitoring circuit carries out charging when a voltage is applied in a state that the switch to be monitored is opened, while it carries out discharging toward a ground point of the voltage applying unit or a microcomputer when the voltage is not applied. In the switch monitoring circuit in which the application of the voltage is ON/OFF controlled by the strobe pulse, accordingly, the charging and the discharging are repeated by the capacitor every cycle of the strobe pulse. Therefore, the consumed current of the capacitor is increased so that the dark current (idling current) of the vehicle is increased.
On the other hand, in recent years, a design corresponding to a formation into a module is carried out in order to shorten a time required for the design related to a vehicle, to easily change the design and to decrease a wire harness, resulting in a reduction in a cost. In this case, for example, a switch belonging to each driving portion such as a door mirror or a door lock is monitored by one microcomputer. More specifically, only one microcomputer monitors the switch as seen from a specific switch. Thus, the microcomputer for monitoring the switch is provided in each driving portion. As compared with the case that some microcomputers monitor switches provided in the whole vehicle through the wire harness in a related technique, therefore, the total number of microcomputers to be mounted on the vehicle is increased.
In a vehicle in which components are formed into a module, accordingly, the number of the microcomputers to be mounted is increased so that the number of capacitors for taking a countermeasure against a static electricity which are to be provided is also increased. As a result, a dark current to be generated on the vehicle in a sleep state is increased still more as compared with a vehicle in which components are not formed into a module. In some cases, therefore, a battery is dead so that the vehicle cannot be driven.